The present invention relates to an exhaust emission control system for an internal combustion engine, and more particularly to an exhaust emission control system including a NOx (nitrogen oxide) removing device for removing NOx and having a function of determining deterioration of the NOx removing device.
When the air-fuel ratio of an air-fuel mixture to be supplied to an internal combustion engine is set in a lean region with respect to a stoichiometric ratio, the emission amount of NOx tends to be increased. To cope with this, a known technique for exhaust emission control includes providing a NOx removing device containing a NOx absorbent for absorbing NOx in an exhaust system of the engine. The NOx absorbent has such a characteristic that when the air-fuel ratio is set in a lean region with respect to the stoichiometric ratio and the oxygen concentration in exhaust gases is therefore relatively high (the amount of NOx is large) (this condition will be hereinafter referred to as xe2x80x9cexhaust lean conditionxe2x80x9d), the NOx absorbent absorbs NOx. When the air-fuel ratio is set in a rich region with respect to the stoichiometric ratio and the oxygen concentration in exhaust gases is therefore relatively low (this condition will be hereinafter referred to as xe2x80x9cexhaust rich conditionxe2x80x9d), the NOx absorbent discharges the absorbed NOx. The NOx removing device containing this NOx absorbent is configured so that NOx discharged from the NOx absorbent in the exhaust rich condition is reduced by HC and CO and then exhausted as nitrogen gas, while HC and CO are oxidized by NOx and then exhausted as water vapor and carbon dioxide.
There is naturally a limit to the amount of NOx that can be absorbed by the NOx absorbent, and this limit tends to decrease with deterioration of the NOx absorbent. A technique of determining a degree of deterioration of the NOx absorbent is known in the art (Japanese Patent Laid-open No. Hei 10-299460). In this technique, two oxygen concentration sensors are arranged upstream and downstream of the NOx removing device, and air-fuel ratio enrichment for discharging the NOx absorbed by the NOx absorbent is carried out. Then, the degree of deterioration of the NOx absorbent is determined according to a delay time period from the time when an output value from the upstream oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio to the time when an output value from the downstream oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio.
However, in the case that a catalyst for purifying exhaust gases is provided upstream of the upstream oxygen concentration sensor, the transient characteristic of the output from the upstream oxygen concentration sensor upon enrichment of the air-fuel ratio changes according to the degree of deterioration of the catalyst (in other words, the transient characteristic of an oxygen concentration on the downstream side of the catalyst changes). Accordingly, when the above-mentioned conventional technique is applied as it is, the accuracy of the deterioration determination is reduced.
That is, as the catalyst upstream of the upstream oxygen concentration sensor becomes older (the degree of deterioration of the catalyst becomes larger), the slope of a change in the output from the upstream oxygen concentration sensor in the case of executing the air-fuel ratio enrichment becomes larger. Further, there is a tendency that as the upstream catalyst becomes older (the degree of deterioration of the catalyst becomes larger), the delay time period from the time the output from the upstream oxygen concentration sensor has exceeded a predetermined threshold to the time the output from the downstream oxygen concentration sensor exceeds the predetermined threshold, becomes shorter. Accordingly, the delay time period in the case that a new catalyst is provided upstream of a deteriorated NOx removing device becomes substantially equal to the delay time period in the case that an old catalyst is provided upstream of a normal NOx removing device, so that there is a case that it is difficult to distinguish between the deteriorated NOx removing device and the normal NOx removing device.
It is accordingly an object of the present invention to provide an exhaust emission control system which can accurately determine the deterioration of a NOx removing device irrespective of the degree of deterioration of a catalyst provided upstream of the NOx removing device.
In accordance with the present invention, there is provided an exhaust emission control system for an internal combustion engine, having a catalyst provided in an exhaust system of the engine for purifying exhaust gases, and a NOx removing device provided downstream of the catalyst for absorbing NOx contained in the exhaust gases in an exhaust lean condition. The exhaust emission control system comprises a first oxygen concentration sensor provided between the catalyst and the NOx removing device for detecting an oxygen concentration in the exhaust gases, a second oxygen concentration sensor provided downstream of the NOx removing device for detecting an oxygen concentration in the exhaust gases, an air-fuel ratio switching module for switching an air-fuel ratio of an air-fuel mixture to be supplied to the engine from a lean region to a rich region with respect to a stoichiometric ratio, a first measuring module for measuring a first time period as an elapsed time period from the time the output from the first oxygen concentration sensor has reached a first reference value after switching the air-fuel ratio from the lean region to the rich region, a second measuring module for measuring a second time period as an elapsed time period from the time the output from the first oxygen concentration sensor has reached a second reference value corresponding to a richer air-fuel ratio with respect to the first reference value and a deterioration determining module for determining whether the NOx removing device is normal or deteriorated according to the first and second time periods and the output from the second oxygen concentration sensor.
With this configuration, the air-fuel ratio is switched from the lean region to the rich region by the air-fuel ratio switching module. Thereafter, the first time period is measured by the first measuring module. Further, the second time period is measured by the second measuring module. Then, the deterioration of the NOx removing device is determined according to the first and second time periods measured above and the output from the second oxygen concentration sensor. The relation between the second time period and the output from the second oxygen concentration sensor is less susceptible to the degree of deterioration of the catalyst provided upstream of the NOx removing device, and the relation between the first time period and the output from the second oxygen concentration sensor is less susceptible to variations in response characteristics of the oxygen concentration sensors. Accordingly, by taking the first and second time periods into consideration, accurate determination of deterioration can be performed.
The deterioration determining module determines that the NOx removing device is normal if the first time period is greater than or equal to an OK determination threshold at the time the output from the second oxygen concentration sensor has reached the first reference value.
The deterioration determining module determines that the NOx removing device is deteriorated if the first time period is less than an NG determination threshold at the time the output from the second oxygen concentration sensor has reached the first reference value.
The deterioration determining module determines that the NOx removing device is normal if the first time period is greater than or equal to an NG determination threshold and less than an OK determination threshold, which is greater than the NG determination threshold at the time the output from the second oxygen concentration sensor has reached the first reference value, and if the second time period is greater than or equal to a predetermined determination threshold at the time the output from the second oxygen concentration sensor has reached the second reference value.
The deterioration determining module determines that the NOx removing device is deteriorated if the first time period is greater than or equal to an NG determination threshold and less than an OK determination threshold, which is greater than the NG determination threshold at the time the output from the second oxygen concentration sensor has reached the first reference value, and if the second time period is less than a predetermined determination threshold at the time the output from the second oxygen concentration sensor has reached the second reference value.
The deterioration determining module determines that the NOx removing device is deteriorated if the output from the second oxygen concentration sensor is greater than the first reference value at the time the first time period has reached an NG determination threshold.
The deterioration determining module determines that the NOx removing device is normal if the output from the second oxygen concentration sensor is less than or equal to the first reference value at the time the first time period has reached an OK determination threshold.
The deterioration determining module determines that the NOx removing device is normal if the output from the second oxygen concentration sensor is greater than the first reference value at the time the first time period has reached an OK determination threshold, and if the output from the second oxygen concentration sensor is less than or equal to the second reference value at the time the second time period has reached a predetermined determination threshold.
The deterioration determining module determines that the NOx removing device is deteriorated if the output from the second oxygen concentration sensor is greater than the first reference value at the time the first time period has reached an OK determination threshold, and if the output from the second oxygen concentration sensor is greater than the second reference value at the time the second time period has reached a predetermined determination threshold.
The present invention also provides an exhaust emission control system for an internal combustion engine, having a catalyst provided in an exhaust system of the engine for purifying exhaust gases, and a NOx removing device provided downstream of the catalyst for absorbing NOx contained in the exhaust gases in an exhaust lean condition. The exhaust emission control system comprises a first oxygen concentration sensor provided between the catalyst and the NOx removing device for detecting an oxygen concentration in the exhaust gases, a second oxygen concentration sensor provided downstream of the NOx removing device for detecting an oxygen concentration in the exhaust gases, an air-fuel ratio switching module for switching the air-fuel ratio of an air-fuel mixture to be supplied to the engine from a lean region to a rich region with respect to a stoichiometric ratio, a first reducing-component amount calculating module for calculating a first reducing-component amount which is an amount of reducing components flowing into the NOx removing device from the time the output from the first oxygen concentration sensor has reached a first reference value after switching the air-fuel ratio from the lean region to the rich region, a second reducing-component amount calculating module for calculating a second reducing-component amount which is an amount of reducing components flowing into the NOx removing device from the time the output from the first oxygen concentration sensor has reached a second reference value corresponding to a richer air-fuel ratio with respect to the first reference value, and a deterioration determining module for determining whether the NOx removing device is normal or deteriorated according to the first and second reducing-component amounts and the output from the second oxygen concentration sensor.
The deterioration determining module determines that the NOx removing device is normal if the first reducing-component amount is greater than or equal to an OK determination threshold at the time the output from the second oxygen concentration sensor has reached the first reference value.
The deterioration determining module determines that the NOx removing device is deteriorated if the first reducing-component amount is less than an NG determination threshold at the time the output from the second oxygen concentration sensor has reached the first reference value.
The deterioration determining module determines that the NOx removing device is normal if the first reducing-component amount is greater than or equal to an NG determination threshold and less than an OK determination threshold, which is greater than the NG determination threshold at the time the output from the second oxygen concentration sensor has reached the first reference value, and if the second reducing-component amount is greater than or equal to a predetermined determination threshold at the time the output from the second oxygen concentration sensor has reached the second reference value.
The deterioration determining module determines that the NOx removing device is deteriorated if the first reducing-component amount is greater than or equal to an NG determination threshold and less than an OK determination threshold, which is greater than the NG determination threshold at the time the output from the second oxygen concentration sensor has reached the first reference value, and if the second reducing-component amount is less than a predetermined determination threshold at the time the output from the second oxygen concentration sensor has reached the second reference value.
The deterioration determining module determines that the NOx removing device is deteriorated if the output from the second oxygen concentration sensor is greater than the first reference value at the time the first reducing-component amount has reached an NG determination threshold.
The deterioration determining module determines that the NOx removing device is normal if the output from the second oxygen concentration sensor is less than or equal to the first reference value at the time the first reducing-component amount has reached an OK determination threshold.
The deterioration determining module determines that the NOx removing device is normal if the output from the second oxygen concentration sensor is greater than the first reference value at the time the first reducing-component amount has reached an OK determination threshold, and if the output from the second oxygen concentration sensor is less than or equal to the second reference value at the time the second reducing-component amount has reached a predetermined determination threshold.
The deterioration determining module determines that the NOx removing device is deteriorated if the output from the second oxygen concentration sensor is greater than the first reference value at the time the first reducing-component amount has reached an OK determination threshold, and if the output from the second oxygen concentration sensor is greater than the second reference value at the time the second reducing-component amount has reached a predetermined determination threshold.